Studies are directed towards understanding the relationship between serum thyroxine (T4) and triiodothyronine (T3) binding, and hormonal delivery to tissues. A novel computer based method of kinetic analysis is employed to analyze tracer disappearance in terms of recirculation times for exchangeable T4 and T3. Comparative studies of charge distribution requirements for nuclear and serum iodothyronine binding are being carried out, and a possible effect of the nuclear receptor complex on nonhistone protein phosphorylation is being sought.